Presently, electric power energy systems are important social infrastructures which should not be at rest even for a moment. Accordingly, stabilization and control of load voltage are important.
Since there is a possibility that sound operation of equipment become damaged by a short-time voltage drop due to a short-time overcurrent such as rush current at lighting an incandescent lamp, starting rush of an induction motor, and saturated rush current at initial excitation rush of a transformer, high voltage is supplied at a supply side in an electric power system.
As a measure against voltage drops of distribution lines at the time of maximum load, an electric power supply system tends to supply voltage excessively by several percents. However, since being at the maximum load is less frequent normally, the voltage over a rated voltage is consumed unnecessarily in many cases. As a result, illumination with a non-inverter fluorescent lamp, a mercury-vapor lamp, a sodium-vapor lamp and the like provides brightness more than necessary. These electric-discharge lamps can be simply dimmed as energy-saving by appropriately decreasing input voltage in a continuous manner. Further, electric power efficiency of a general induction motor drops due to increasing of iron loss. With a small induction motor, it is well known that the motor efficiency is increased by decreasing load voltage to be slightly lower than a rated voltage when being operated at a load rate at a degree of 70% or less.
Typically, it is general that appropriate adjustment of alternating voltage is performed by tap-changing of a transformer. However, in the case of a mechanical type, there have been problems that the voltage output by the tap-changing is step-shaped and that time delay of operation occurs. Further, a slide transformer (i.e., a variable transformer) is expensive and has a problem with durability. Since a back-to-back method of an inverter/converter does not require frequency changing, it is considered that the adoption thereof increases cost and enlarges electric power loss.
Further, in a direct-current circuit, the voltage is controlled to be constant by a direct-current voltage adjusting circuit after electric power is converted from alternating-current to direct-current. As a technology to perform the same thing as the above at an alternating-current side, a magnetic amplifier utilizing ferroresonance existed in the past. However, little development has been made thereafter. An alternating voltage controller utilizing a thyristor has drawbacks that a current waveform is distorted and a lagging power factor of current (i.e., a state that current is lagged behind voltage) is caused as a result of voltage control. With a lagging power factor load such as an inductive load, it is also a problem that a large voltage noise is caused due to occurrence of high voltage at the time of voltage interruption.
Further, on the other hand, a circuit technology called a magnetic energy recovery switch (hereinafter, called “MERS”) was proposed and already granted as a patent (see patent document 1).
The MERS utilizes a switching circuit or a semiconductor element without having reverse blocking capability, that is, being a reverse conduction type. For example, the switching circuit or the semiconductor element of the reverse conduction type adopt a circuit constituted with a self-turn-off device and a diode while a positive side of the self-turn-off device and a negative side of the diode is connected and a negative side of the self-turn-off device and a positive side of the diode is connected (hereinafter, simply called “reversely parallel” connection), a semiconductor element such as a power MOSFET incorporating a parasitic diode when manufacturing, or the like. In the following, the switching circuit or the semiconductor element of the reverse conduction type are simply called “reverse conduction type semiconductor switch”.
The MERS includes a full-bridge circuit constituted with a first reverse conduction type semiconductor switch leg forming a first alternating-current terminal at a connecting point between a negative side of a self-turn-off device constituting a first reverse conduction type semiconductor switch (hereinafter, simply called “ the negative side of the reverse conduction type semiconductor switch”) and a positive side of a self-turn-off device constituting a second reverse conduction type semiconductor switch (hereinafter, simply called “the positive side of the reverse conduction type semiconductor switch”) and a second reverse conduction type semiconductor switch leg forming a second alternating-current terminal at a connecting point between a negative side of a third reverse conduction type semiconductor switch and a positive side of a fourth reverse conduction type semiconductor switch, having a positive terminal formed by connecting both positive sides of the first reverse conduction type semiconductor switch and the third reverse conduction type semiconductor switch and having a negative terminal formed by connecting both negative sides of the second reverse conduction type semiconductor switch and the fourth reverse conduction type semiconductor switch, and a capacitor connected between the positive terminal and the negative terminal of the full-bridge circuit.
A circuit being an object to be controlled by the MERS is connected between the first alternating-current terminal and the second alternating-current terminal of the full-bridge circuit.
Here, the first reverse conduction type semiconductor switch and the fourth reverse conduction type semiconductor switch are assumed to be a first pair and the second reverse conduction type semiconductor switch and the third reverse conduction type semiconductor switch are assumed to be a second pair. By controlling ON/OFF states of the reverse conduction type semiconductor switches so that self-turn-off devices constituting the two reverse conduction type semiconductor switches of the second pair are to be in a blocked state (hereinafter, simply called “an OFF state of the reverse conduction type semiconductor switch”) when self-turn-off devices constituting the two reverse conduction type semiconductor switches of the first pair are in a conductive state (hereinafter, simply called “an ON state of the reverse conduction type semiconductor switch”) and the second pair is to be in an ON state when the first pair is in an OFF state, the MERS functions as a switch circuit of bidirectional current in which the capacitor is capable of absorbing “snubber energy” accumulated throughout the full-bridge circuit and the control object circuit and regenerating to the control target circuit when the current of the circuit is interrupted. The direction of current passing through the control target circuit can be switched between the forward direction and the reverse direction corresponding to a purpose and a range of the control.
When a circuit serially connecting an inductive load as a control target circuit and an alternating-current power source between the first alternating-current terminal and the second alternating-current terminal of the MERS is utilized, alternating-current power to be supplied to the inductive load can be controlled. It is actualized that the capacitor absorbs the “magnetic energy” accumulated at an inductance component of the inductive load (i.e., the capacitor is charged) and regenerates to the inductive load (i.e., the capacitor is discharged) due to resonance between the capacitor and the inductance component of the inductive load. The above was proposed as an alternating-current power source apparatus utilizing an MERS and already granted as a patent (see patent document 2).
In the alternating-current power source apparatus utilizing the MERS, electrostatic capacity of the capacitor being a capacity to be in a resonance state with the inductance of the inductive load is selected corresponding to a purpose and a range of control. In particular, by selecting the electrostatic capacity of the capacitor so that resonance frequency determined by the electrostatic capacity of the capacitor and the inductance of the inductive load is equal to or higher than the switching frequency of the reverse conduction type semiconductor switch, it is possible to perform soft switching operation as the self-turn-off device constituting the reverse conduction type semiconductor switch is at approximate zero voltage and zero current when the reverse conduction type semiconductor switch is turned on and the self-turn-off device constituting the reverse conduction type semiconductor switch is at approximate zero voltage when the reverse conduction type semiconductor switch is turned off.
In the alternating-current power source apparatus utilizing the MERS, the ON/OFF states of the reverse conduction type semiconductor switches are controlled so that the second pair of the reverse conduction type semiconductor switches is to be in an OFF state when the first pair is in an ON state and the second pair is to be ON state when the first pair is in an OFF state. The time ratio (i.e., the duty ratio) of ON time and OFF time of the reverse conduction type semiconductor switches is 0.5, that is, the ON time and the OFF time are the same. When time-base indication of the ON/OFF states of the reverse conduction type semiconductor switch is assumed to be a control signal, the control is performed so that a control signal phase is synchronized with a voltage phase of the alternating-current power source and the control signal phase is advanced from the voltage phase of the alternating-current power source (i.e., being in a state that variation of the control signal phase is advanced in terms of time). The alternating-current electric power to be supplied to the inductive load can be controlled by varying phase difference between the control signal phase and the voltage phase of the alternating-current power source corresponding to a purpose and a range. Further, it is characteristic that the supplying voltage to the inductive load can be heightened by advancing the current phase and the supplying voltage to the inductive load can be also lowered by drastically advancing the current phase.
In the alternating-current power source apparatus utilizing the MERS, when a power factor is improved by the MERS in the case with a load of a lagging power factor such as an inductive load, there has been a fear that the inductive load is damaged as the supplying voltage to the inductive load becomes overvoltage. In order to address the above, the present inventor proposed an alternating voltage control apparatus to make the power factor of the power source current to be 1 as a whole by supplying voltage lower than the voltage of the alternating-current power source to the inductive load as further drastically advancing the phase of current to the inductive load and by mating with current of a lagging power factor of another inductive load to which an MERS is not connected (hereinafter, simply called “the alternating voltage control apparatus with advanced-phase current”). The proposal was laid-open and already publicly known (see patent document 3).
[Patent document 1] Japanese Patent No. 3634982
[Patent document 2] Japanese Patent No. 3735673
[Patent document 3] Japanese Patent Application Laid-open No. 2007-058676